This invention relates to a process for the preparation of castable polyurea elastomers from aliphatic diisocyanates, amine-terminated polymers, and aromatic diamine or polyamine chain extenders in a single step.
The casting of polyurethane elastomers in open molds is known. E.g., A. Awater, "PU cast elastomers" in Polyurethane Handbook, ed. G. Oertel (New York: Hanser Publishers, 1985), pages 372-388; J. H. Saunders and K. C. Frisch, "Urethane Elastomers" in Polyurethanes. Chemistry and Technology (New York: Interscience Publishers, 1962), pages 273-314. More particularly, the casting of polyurethane elastomers from diisocyanates, polyoxyethylene polyols, and low molecular weight diols or polyols is known. For example, U.S. Pat. No. 3,620,905 discloses optically clear, light-stable polyurethanes prepared from 4,4'-methylenebis(cyclohexylisocyanate), polyethers or polyesters having a molecular weight of from 500 to 3000, and a low molecular weight diol. However, polyurethane elastomers prepared by these known methods show comparatively poor dimensional stability at elevated temperatures.
Polyurethane/polyurea hybrids based on aliphatic isocyanates and having improved high temperature stability can be cast. Such compounds are prepared in two-step processes by chain extending prepolymers of aliphatic diisocyanates and polyoxyalkylene polyols using low molecular weight diamine chain extenders such as diethyltoluenediamine (also known as "DETDA"). E.g., U.S. Pat. Nos. 4,208,507, 4,581,433, and 4,927,901. However, it is often necessary to use very specific reactive components, as well as special additives, to obtain suitable results. E.g., U.S. Pat. Nos. 3,891,606 and 4,587,275.
U.S. Pat. No. 4,569,982 discloses polyurea elastomers prepared by the reaction of aliphatic diisocyanates having melting points greater than 40.degree. C. with aliphatic or aromatic polyamines. However, the aliphatic diisocyanates must be solid at room temperature so that the heterogeneous mixtures do not react until the diisocyanate is melted at elevated temperatures. Liquid diisocyanates are thus not suitable.
It is now been found that castable polyurea elastomers can be prepared in a one-shot process using aliphatic diisocyanates, amine-terminated polymers, and aromatic diamine or polyamine chain extenders. Elastomers prepared according to the invention exhibit improved dimensional stability at elevated temperature relative to known polyurethane elastomers based on aliphatic isocyanates. As a further advantage, it has been found that it is not necessary or even desirable to use catalysts to accelerate the polyurea-forming reaction.